The Translation of “I was recently surprised by this simple thought”

I’m reading a Dual Language book in which the English translation of the Russian is “I was recently surprised by this simple thought”. The original Russian is “Я недавно понял и удивился этой простой мысли”. The thing that confuses me is that I would have expected the word “thought” to be in the instrumental case but as I understand it the instrumental singular ending for feminine words ending in the soft sign is мыслью. Could someone tell me what case мысли is in and why?
Thank you

The verb удивиться is followed by the Dative case, удивиться чему? I think the fact that confuses you is that in English the construction is "to be surprised by", so you think it is a passive construction in Russian, followed by the Instrumental case.